Fluororesins have excellent chemical resistance, non-tackiness, heat resistance, low frictional coefficient, and electrical insulating properties. However, flexible engineering plastics such as PEEK and PPS, which are hard and can be molded into complex shapes by injection molding, instead find frequent commercial use in articles having a complex shape and requiring hardness.
A technique for improving the hardness of a fluororesin involves filling the fluororesin with a large amount of solid filler. Such filled fluororesin can have enhanced hardness, abrasion resistance, low frictional coefficient, and creep resistance due to the filler, in addition to the original chemical resistance, non-tackiness, heat resistance, low frictional coefficient, and electrical insulating properties of the fluororesin. They are therefore used as various members or seal/gasket materials. However, in recent years, there has been a demand for higher hardness, abrasion resistance, low thermal expandability, and creep resistance in sliding members such as piston rings or seal materials for AT/CVT in the automotive field and in gasket materials in the field of chemical plant equipment. However, current commercial embodiments of filled fluororesin compositions are not satisfactory.
Although melt flowable fluororesins have melt-moldability and excellent processability, in addition to excellent chemical resistance, non-tackiness, heat resistance, low frictional coefficient, and electrical insulating properties, the resins themselves are flexible and are therefore unsuitable for the applications described above. A material can be made hard by blending a solid filler into a melt flowable fluororesin to a high filling ratio, but the melt flow characteristics (e.g., melt flow rate (MFR)) of the fluororesin composition at the time of molding are diminished. Therefore, the melt-moldability, which is a feature of thermally meltable melt flowable fluororesins, is sacrificed, and it becomes difficult to develop the substance into an article with a complex shape. On the other hand, by more finely dispersing fillers into the resin, the interactions between the fillers are enhanced, which improves the hardness or creep resistance of the resin composition. It is already known that a fluororesin composition in which inorganic fine particles are dispersed to a high degree may be obtained by means of co-coagulation of a dispersion of thermally meltable fluororesin and a colloidal solution of inorganic fine particles (see, for example, Japanese Unexamined Patent Application Publication No. 2007-119769A). However, the hardness of a fluororesin composition, in which such inorganic fine particles (i.e., the nano-sized silica particles of JP 2007-119769A) are dispersed to a high degree at a primary particle level, is insufficient. In addition, a hard resin can be prepared by filling a thermosetting polyimide (PI) resin with a filler. However, a PI molded by means of compression molding alone requires the machining of the compression-molded product, and the handling greatly differs from that of thermoplastic resins, which can be shaped easily. Moreover, PI is inadequate in that the non-tackiness and low frictional coefficient are inferior to those of fluororesin.
A silica porous medium having mesopores with a uniform honeycomb shape synthesized using the micellar structure of a surfactant as a mold is known as mesoporous silica (MPS). Production of such is described, for example, in Japanese Unexamined Patent Application Publication No. 2002-053773A, or by Inagaki, S.; Fukushima, Y.; Kuroda, K. in J. Chem. Soc., Chem. Commun. 1993, 8, 680. MPS is widely used for absorbents, a carrier for catalysts, drugs, or the like due to the properties of the porous medium. Specific examples are given in Japanese Unexamined Patent Application Publication No. 2011-046888A, in which MPS is used as a filler for a resin composition, include as an organic resin that becomes a matrix, epoxy resins, phenol resins, polyurethanes, polyimides, unsaturated polyesters, vinyl triazine, crosslinked polyphenylene oxide, and curable polyphenylene ethers. In an embodiment serving as a mode for carrying out the invention, only epoxy resins are described, and there is no mention of fluororesins.
Fluororesins, in particular perfluororesins such as tetrafluoroethylene-perfluoro(alkylvinylether) copolymers (herein also referred to as PFA) and tetrafluoroethylene-hexafluoropropylene copolymers (herein also referred to as FEP), have very low surface energy. Therefore, their affinity to silica is los, as silica has high surface energy. In Japanese Unexamined Patent Application Publication No. 2011-046888A, MPS is subjected to surface hydrophobizing treatment, but the affinity with fluororesins remains low, and the filling of fluororesin into the MPS pores does not occur. In addition, in Japanese Unexamined Patent Application Publication No. 2005-163006A, and Japanese Unexamined Patent Application Publication No. 2004-311326A, composite materials consisting of silica and fluororesins are disclosed, and it is described that these materials can be used as circuit board materials for high-frequency signals. However, these fluororesin compositions are intended to be used as circuit board materials, and the silica pores are not filled with fluororesins since they reduce the dielectric constant. Therefore, these fluororesin compositions do not have the high hardness required for sliding materials, heat-resistant seals, or gasket materials.